The chemical effects of ultrasound in aqueous solutions and organic liquids are being studied as model systems. Ultrasound is used to produce hyperthermia in radiation therapy and a new approach, sonodynamic cancer therapy, seeks to take advantage of the synergistic effects of ultrasound and anticancer drugs (eg., adriamycin). Electron spin resonance and spin trapping methods were used to investigate the sonochemistry and antioxidant properties of ubiquinone (Coenzyme Qo) in aqueous solutions. Evidence for the formation of superoxide radicals in oxygen-free argon-saturated aqueous solutions was obtained since Superoxide Dismutase (SOD) inhibits the sonochemical formation of ubiquinol. Spin trapping studies with 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) established the formation of 5-yl radicals by addition of OH radicals to the 5,6 double bond of ubiquinone. Pyrolysis products such as methyl radicals were also formed. Conclusive evidence for the formation of superoxide radicals formed by 50 kHz sonolysis of oxygen free argon-saturated water was obtained by means of the SOD-inhibited reduction of cytochrome c. Superoxide is formed by radical reactions as a result of the very high temperatures (approx. 5000 K) in collapsing cavitation bubbles. 2,4,6-tri-t. butylnitrosobenzene (3tBNB) was found to be a reliable spin trap for investigating the sonolysis of organic liquids. Thus, similar results were found in the sonolysis of dimethylformamide using DBNBS and 3tBNB as the spin traps. The sonolysis of a variety of organic liquids, including n-alkanes, n-alcohols, dioxane, cyclohexane and toluene were investigated. In the homologous series of n-alkanes and n-alcohols the spin adduct yields with 3tBNB increase with decreasing vapor pressure as predicted from the thermal hot-spot theory of sonochemistry.